1. Field of the Invention
This invention relates to a socket for electric parts which is used for the testing of electric parts, such as IC packages, and a mounting to a circuit substrate.
2. Description of the Prior Art
What is called an open-top type socket for electric parts has been known in the past, and its structure is rich in variety. This socket is equipped with a socket body having a mounting face for mounting an electric part thereon; numerous contact pins arranged in the socket body, each having a base portion, a contact portion, a trigger portion, and an arcuate spring portion; and a cover put on the socket body, free to move up and down. Each contact pin is moved outwardly against its biasing force through the trigger portion pushed by the downward pressure of the cover so that the electric part can be mounted on the mounting face from above the cover. By releasing the pressure from the cover pressed down, the contact pin is returned to its original position to press the contact portion of the contact pin against each of leads of the electric part placed on the mounting face of the socket body.
FIG. 1 is a left-hand sectional side view showing an example of the socket mentioned above. In this figure, reference numeral 1 denotes a socket body of rectangular plane shape, having a guide groove 1a piercing the socket body vertically and a mounting base 1b; 2 denotes a cover of bilaterally symmetrical frame shape, having a pillar 2a inserted to slide freely in the guide groove la and a cam 2b along which a part of each of contact pins described later slides; 3 denotes a spring imparting the biasing force of an upward movement to the cover 2; 4 denotes contact pins, each having a contact portion 4a, a trigger portion 4b sliding along the cam 2b of the cover 2, an arcuate spring portion 4c, a base portion 4d interposed between respective two of numerous insulating ribs configured in the socket body 1, and connecting terminals 4e inserted and fixed in holes bored through the socket body 1; and P denotes an electric part, such as an IC package, having leads L which are mounted on the mounting base 1b of the socket body 1 upon loading and each of which comes in contact with the contact portion 4a of the contact pin 4. Also, the cover 2 is constructed so that its upward movement is limited to the position shown in the figure by a locking mechanism, not shown, placed between the socket body 1 and the cover 2.
The loading of the electric part P at the position shown in the figure and its unloading therefrom are performed by pressing down the cover against the resilience of the spring 3. Specifically, when the cover is pressed down from the position of the figure, the trigger portion 4b is displaced outwardly, by the cam 2b, to the position indicated by a chain line in FIG. 1, against the resilience of the spring portion 4c. At the same time, the contact portion 4a is moved outwardly to the position indicated by a corresponding chain line to retire from the position held by each lead L of the electric part P.
This outward movement for the retirement of the contact portion 4a is also performed in cacti of the right-hand contact pins, not shown, arranged symmetrically with respect to the contact pins 4, and thus in this state, it becomes possible that the electric part P is mounted on, or dismounted from, the mounting base 1b. Subsequently, the downward pressure of the cover 2 is released, and thereby the cover 2 and the contact pin 4 are returned to the positions indicated by solid lines in virtue of the restoring force of the spring 3 and its own biasing force, respectively. In this way, the loading or unloading of the electric part P is completed.
In the conventional socket for electric parts, as mentioned above, the trigger portion 4b of the contact pin 4 is in general located away from the center of the socket body 1, that is, close to the outer side of the socket body 1. This position of the trigger portion 4b is disadvantageous when the socket body is made small in size. Thus, to compactly design the socket body, it is considered that the trigger portion is located closer to the center of the socket body. In this case, however, it is imperative that the center of turning of the contact pin 4 is situated nearly below the center of the socket body 1 because of its inner structure. Hence, the problem is produced that an open angle of the contact portion 4a where the cover 2 is pressed down becomes smaller, and when the downward pressure of the cover 2 is released to return each contact pin to its original position, the contact portion 4a catches in each lead L of the electric part P.
Furthermore, in order to improve the stability of contact between the contact portion 4a of each contact pin 4 and the lead L of the electric part P, the socket for electric parts of this type requires a wiping action, as it is usually called, to slide the contact portion 4a along the lead L, upon loading. In order to increase the efficiency of this wiping action, it is necessary to minimize an angle of advance .theta. (FIG. 2) made by the contact portion 4a with the lead L when the downward pressure of the cover 2 is released. However, if the angle of advance .theta. is made small, a case may crop up in which the contact portion 4a strikes the end face of the lead L (see FIG. 2) and is not smoothly seated on the lead L.
In the shop in which the electric parts like IC packages are mass-produced, where the socket of this type is used to carry out the performance tests of produced electric parts, the operations of the downward pressure of the cover 2 and the release therefrom are repeatedly performed and consequently, the contact pin 4 is repeatedly oscillated between the positions indicated by the solid and chain lines. In general, the contact pin 4 is constructed of thin electrically conductive material such as beryllium copper, and thus is liable to produce a stress, particularly in the arcuate spring portion 4c, because of the repeated oscillating motion of the contact pin 4.
For each contact pin 4 used in the conventional socket, the arcuate spring portion 4c is constant in width. Thus, in each of the conventional contact pins, since stress concentration is liable to occur in a particular part of the arcuate spring portion 4c, the fracture rate of the arcuate spring portion 4c is extremely high. Even though it is not fractured, the spring force of the arcuate spring portion 4c will be reduced, and thus the pressure or contact between the arcuate spring portion 4c and each lead L of the IC package will be decreased, frequently causing inconvenience to the examination and performance test of the electric part.
FIG. 3 shows the result of the simulation of stress distribution in the conventional contact pin. High stresses are concentrated in regions R1 and R2 of hatching portions. The stress concentration is brought about along the arcuate spring portion 4c, and tends to increase in going from a connection between the contact portion 4a and the trigger portion 4b to the base portion 4d.